Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. This technique is commonly used for plastics, such as to join dissimilar materials. The technique is used in industries such as automotive, appliance, electronic, toy, packaging, textile, and medical, among others.
Energy directors are raised portions of material molded, set, affixed, or otherwise in contact with or positioned adjacent to a surface of a workpiece used during ultrasonic welding. Energy directors concentrate ultrasonic energy to rapidly initiate softening and melting of surfaces at a joining interface.
FIG. 1 illustrates a conventional process of ultrasonic welding to bond a first workpiece 110 with a second workpiece 120. As shown in step 100, the first workpiece 110 is positioned in contact with the second workpiece 120, which includes a plurality of energy directors 130. A welding tip 150 is positioned near an upper surface of the first workpiece 110. The welding tip 150 typically has a beveled (e.g., chamfered) edge designed to contacts the upper surface of the first workpiece 110.
At step 106, the welding tip 150 contacts the upper surface of the first workpiece 110, creating an energy transfer area 160, initially formed by a contact area between the beveled edge of the welding tip 150 and the upper surface of the first workpiece 110. Each of the energy directors 130 directly adjacent the energy transfer area 160 begins to melt at the same time due to ultrasonic vibrations from the welding tip 150 passing through the energy transfer area 160 and generating heat within each of the energy directors 130. Ultimately, the energy transfer area 160 may expand through some or all of the first workpiece 110. Additionally the energy transfer area 160 may expand through some or all of the energy directors 130 and/or the second workpiece 120.
FIG. 2 illustrates a perspective view of the conventional ultrasonic welding process. Specifically, FIG. 2 illustrates the grandness of the energy transfer area(s) 160 as the welding tip 150 travels along a directed path, illustrated by an arrow. The welding tip 150 may travel continuously or in discrete intervals across the first workpiece 110. Where discrete intervals are used, the welding tip 150 (i) forms a weld within a first weld area, (ii) raises from the first weld area, (iii) travels to a second weld area, and (iv) lowers to form a second weld within the second weld area.
Referring back to FIG. 1, at step 109, the workpieces 110 and 120 join to form a weld having a weld width 170. As shown, air pockets 180 are formed within the weld width 170. When the energy directors 130 melt, air is trapped within the weld, forming the air pockets 180 shown developing at step 106 and present in the final product at step 109. Air pockets 180 weaken strength of the weld and can lead to premature failure.